Type 2 diabetes is the most common chronic disease in the US, effecting 8% of the population. Currently, diabetes is usually diagnosed after a mean delay of 7 years, often after the development of complications. The DPP is designed to determine whether active interventions (intensive life-style or metformin) can prevent or delay the development of non-insulin dependent diabetes mellitus (NIDDM) in individuals who are at high risk by virtue of having Impaired Glucose Tolerance (IGT). Secondary goals include reduction of macrovascular disease and risk factors that accompany IGT and NIDDM. Recruitment began in 1996 and will end in mid-1999, with more than 3,600 recruited subjects. Subjects will be followed for a mean of 4.5 years. The investigators have screened more than 8,000 potential volunters at the MGH DPP Center, performed more than 1,200 OGTT's (Step 2 recruitment) and have randomized 134 subjects as of January, 1999.